Revision History for Scientific Investigations Report 2012-5212 Elizabeth A. Murphy, Jennifer B. Sharpe, and David T. Soong Ohio River Backwater Flood-Inundation Maps for the Saline and Wabash Rivers in Southern Illinois ----------------------------------------------------- Posted online December 5, 2012 ----------------------------------------------------- Revised and reposted September 18, 2014, version 1.1 The inundation surfaces, kmz, Readme, and metadata files were revised. Newly-available, high-resolution Light Detection and Ranging (lidar) data were used to generate new inundation surfaces. The new inundation surfaces are available at a USGS Web portal at http://wim.usgs.gov/FIMI/FloodInundationMapper.html#. The new surfaces are also available in the Downloads Directory of the report page. New Readme and Metadata files have been posted for the new surfaces. New surfaces were generated for the study area by representing the topography with a new Digital Elevation Model (DEM) created from lidar data for the study area in Illinois and the 30-meter National Elevation Dataset (NED) for a small part of the mapped area in Kentucky. The original DEM for the study was created from 30-meter NED data. The lidar was flown in 2011 by Woolpert Inc. under a Federal Emergency Management Agency (FEMA) contract and quality-assured/quality-checked by USGS National Geospatial Program staff (Rolla, Missouri) in 2013. The lidar-derived DEM was originally processed to represent the elevation of the Ohio River water surface at the time of the lidar data collection. When the inundation surfaces were first generated with the lidar-derived DEM, some locations in the river were being mapped as not covered by water. To correctly represent the Ohio River as being inundated within its banks, the lidar-derived DEM was reprocessed to represent the area between the banks of the Ohio River as an elevation of 335 feet North American Vertical Datum of 1988 (NAVD88). The inundation surfaces were then regenerated with the reprocessed lidar-derived DEM data. As in the original study, the inundation surfaces were not edited to remove ponded areas disconnected from the riverine flooding nor to represent whether the elevation of the water was above or below the elevation of the bridge decks. Therefore the map should not be used to determine whether a bridge will be flooded with water at a given water surface elevation. The lidar metadata indicates that a group of tiles in the dataset “lacked sufficient ground control points to properly delineate or hydro flatten” them. This lack of ground control resulted in a section of the study area not mapping consistently with the surrounding area for the stages of 31-39 feet. The affected areas are identified on the mapper. The table below shows the difference in inundated area (in square miles) between the surfaces generated with the original NED DEM and the surfaces generated with the new lidar DEM. It also shows the percent change between the NED-generated surfaces and the lidar-generated surfaces. The table includes only the surfaces which were generated from both the NED and lidar DEMs. The differences are small for stages 41, 51, and 61 ft (the inundated area depicted by using the lidar DEM is 6-8 percent smaller than the area depicted with the original NED DEM at 41 and 61 ft and 2 percent larger at 51 ft); however, the percentage reduction between the original NED DEM area and the lidar DEM is large for the 31 ft stage surface, likely because of the area where the lidar had poor ground control. The lidar and NED-generated surfaces were also compared visually and the inundated areas appeared to be similar overall. Original New NED lidar Percent DEM DEM change Elevation STAGE (ft above Square Square (ft) NAVD88) miles miles 31 340 59.23 42.85 -28 41 350 186.86 175.19 -6 51 360 304.67 311.99 2 61 370 528.24 484.36 -8 -------------------------------------------------------------------